Take your shotgun to a shooting range where variable target distances are available.
Start at a shorter-range target, about 25 yards. Fire a three round group, aiming carefully at the target's bullseye. Then examine your target.
Find the rough center of the group (within 9 inches or so). If the shots seem to be scattered randomly, take more shots, firing carefully at the bulls eye each time, until you get a good sense of where the center of your group is.
Measure the distance from the center of your group to the bulls eye. Note how far above or below and how far left or right your weapon is shooting.
Adjust your scope to compensate for any offset. Scopes sometimes vary, but usually one "click" of the adjustment knobs, or about ten degrees of turn, accounts for approximately one inch at 100 yards. At 25 yards, you'll usually use about a quarter-turn of each knob to adjust for one inch at the target.
Shoot another group at the 25-yard target and check the results. Repeat and adjust until you are satisfied.
Move on to a farther distance when you are satisfied with the sighting at 25 yards, either 50 or 75 yards. Most shotgun shooters do not shoot beyond this distance very often, but you can practice at up to 100 yards if you wish. The most important part of sighting in your shotgun is getting comfortable with the gun and the sighting yourself.